This invention relates to the repair of alloy articles, and more particularly, to brazing alloy compositions used in an activated diffusion healing process utilized to repair gas turbine nozzles with service-induced damage.
There are numerous cobalt based alloys used in both the manufacture and repair of high-temperature operating gas turbine engine components including combustors, turbine vanes, nozzles, etc.
Representative examples of cobalt base alloys for the production and/or repair of superalloy articles are disclosed in U.S. Pat. Nos. 4,830,934; 4,614,296; 4,381,944; 4,285,459; 4,058,415; 3,355,287; 3,276,865; 3,271,140; and 4,396,577. Specific known brazing alloy compositions are set forth below in Table I.
During operation of such components under high-temperature operating conditions, various types of damage or deterioration can occur. For example, cracks can result from thermal cycling, foreign object impact, and/or corrosion. It is, of course, well known that the cost of these components is relatively high and therefore, it is more desirable to repair such components than to replace them.
Typically, surface grinding would be used to remove cracks, corrosion or foreign object damage craters.
An alternate method of turbine nozzle repair and refurbishment to conventional gas tungsten arc or plasma arc weld repair techniques involves the in situ vacuum brazing of a brazing filler alloy in a suitably prepared area. An existing state-of-the-art process known as activated diffusion healing (ADH) has been successfully utilized to effect repairs by depositing brazing filter alloys similar to alloys 1 and 2 found in Table I below below. In practice, a 50/50 volume percent mixture of these alloys is used to make deposits up to 75 mils in depth.
TABLE I __________________________________________________________________________ BRAZING ALLOY COMPOSITIONS (WEIGHT PERCENT) Alloy No. Identity Co. Ni Cr W C B Si Mn Other __________________________________________________________________________ 1 X-40 Bal. 10 25 7.5 0.5 0.015 -- -- 2 D-15 10.3 Bal. 15.3 -- -- 2.3 -- -- 3.5 Al, 3.4 Ta 3 S-57B Bal. 10 21 -- 0.05 3.3 2 -- 2.5 Al, 4.6 Ta, 0.2 4 Amdry 400 Bal. 17 19 4 0.4 0.8 8 -- 5 Amdry 788 Bal. 21 22 14 0.05 2.0 2 -- 0.03 La __________________________________________________________________________
The brazing alloys of this invention are adapted specifically for use with a known Co-base or parent superalloy substrate material, commercially known as FSX-414, having the following composition, by weight:
______________________________________ Ni 10% Cr 29% W 7.5% C 0.25% B 0.015% Si 0.9% Mn 0.6% Co Balance ______________________________________
Referring back to Table I, Alloy #1, known commercially as X-40, is similar in properties to FSX-414, although less corrosion resistant due to its somewhat lower Cr content. It is used as a filler to provide good mechanical properties and does not itself melt during the vacuum brazing cycle. Brazing alloy #2, known commercially as D-15, melts, wets, and flows with excellent brazing characteristics but has very poor hot corrosion resistance and is, therefore, not suitable for industrial gas turbine nozzle service. Other state-of-the-art brazing alloys such as #3, #4, and #5 in Table I are somewhat more corrosion resistant than D-15 but do not provide optimum brazing or mechanical properties nor maximum corrosion resistance.
The present invention relates to an improved brazing alloy composition for the repair of environmentally-damaged gas turbine parts exposed for long periods of time to corrosive high-temperature operation. These brazing alloy compositions are specifically adapted for use in an activated diffusion healing process to rebuild, for example, nozzles, by brazing a mixture of the brazing alloy and the base or parent metal of the nozzle in particulate form. Specifically, repair is effected by brazing a deposit containing a mixture of braze alloy and parent alloy powders. The vacuum brazing cycle causes the braze alloy to melt and alloy with the parent alloy powder and the parent substrate surface. A post-braze diffusion heat treatment cycle is then applied to promote further interdiffusion. The mechanical properties of the resultant repaired region nearly approach those of the parent alloy.
The brazing alloys in accordance with this invention are cobalt-base alloys which, relative to existing commercial alloys, have an increased concentration of elements known to provide surface protection via adherent oxide layers, i.e., Chromium (Cr) and Silicon (Si), while minimizing those elements which may have a deleterious effect on corrosion resistance, i.e., Boron (B) and Tungsten (W). Relatively small concentrations of B and/or Si are used to depress the melting point so that the brazing alloy melts before the parent metal.
The principal objective of the invention is therefore to provide brazing alloy compositions which are balanced to achieve high temperature strength, ductility and corrosion resistance while displaying conventional braze alloy characteristics of low melting temperatures and good wetting and flowability under conventional vacuum brazing conditions.
Accordingly, in one preferred embodiment of this invention, a brazing alloy is provided which comprises, by weight:
______________________________________ Nickel from about 8.5% to about 12.5% Chromium from about 24% to about 40% Tungsten from about 0% to about 9% Carbon from about 0.03% to about 0.6% Boron from about 0.01% to about 3.5% Silicon from about 1.0% to about 11% Manganese up to about 2% Cobalt Balance ______________________________________
The alloys of this invention are high temperature (1200.degree.-1900.degree. F.) resistant and provide superior mechanical properties as well as oxidation and corrosion resistance in the above temperature range.